P-Delta Analysis of a 3D structure contained tension-only cable has unexpected result

[lilylucyleo]

Hello everyone,

I am a new member of STAAD user club. Happy to talk to senior users in the forum.

I am doing a p-delta analysis to a tent, which is steel framed and reinforced by steel tension cables. I tried to use "PERFORM ANALYSIS" only and the maximum steel compliance ratio, as per CSA S16 is around 1.6. But when I tried to use p-delta analysis, the ratio went up to 3.1. So I looked up the steel design details, and found that the MZ moment is 2.18 Knm,while f using "PERFORM ANALYSIS", MZ is around 0.5 KNM.I used "REPEAT LOAD" instead of "LOAD COMBINATIONS".

I think it is due to the additional moment caused by p-delta effect. But I am really confused with such large differences. I have been tried 2D structure with p-delta analysis, the result is 1.7. Also if the tension-only cables were removed, the code compliance ratio will be 1.7, just slightly higher than the conventional analysis.

I attached the snapshot of my 3D tent, the steel design code together with the postprocessing results. I will be very happy if some expert can give me explanation whether the analysis is correct and which ratio should I believe.

The snapshots are:

http://files.engineering.com/getfil...45-a878-0d0aea27173a&file=whole_structure.png

http://files.engineering.com/getfil...78-0216e8173499&file=steel_design_details.png

http://files.engineering.com/getfil...-a8e0-6c47771da394&file=beam_force_detail.png

Thanks a lot!
Fio

 

[Sye123]

Using REPEAT LOAD is necessary for PDELTA type analysis. It is difficult to conclude what exactly is causing the difference without taking a look at the actual model Is it possible for you to upload the model file ( the .std file ) ? You may zip it and then upload it.

 

[JoshPlumSE]

View the deflected shape of the structure with and without P-Delta. How is the deflection changing? That may give you some insight into what is physically happening as a result of P-Delta. Maybe the bottom chord of that roof truss is beginning to buckle laterally.